A thin film including a silver alloy (i.e., Ag-based alloy thin film) has specific properties, such as high reflectivity, low electrical resistivity, and high thermal conductivity. Therefore, the Ag-based alloy thin film is applied to reflective films of an optical recording medium, electrodes and reflective films of a reflection-type liquid crystal display or the like. In particular, since an Ag-based alloy thin film has high reflectivity with respect to a blue-violet laser which is used in a next-generation optical disk and high thermal conductivity which is required for a write-once-read-many/rewritable disk, the Ag-based alloy thin film is advantageously used as a reflective film or a semi-transmissive reflective film of an optical recording medium.
A sputtering method is suitable to form an Ag-based alloy thin film which is used as the above-mentioned reflective film or the like. The sputtering method of producing thin films is performed in the following way. In a vacuum into which Ar (i.e., Argon) has been injected, a plasma discharge is formed between a substrate and a sputtering target (hereinafter, referred to simply as a “target”), and Ar ionized by this plasma discharge is allowed to collide with the target. As a result, atoms of the target are ejected from the target and are deposited on the substrate to produce thin films. The sputtering method can advantageously form thin films more excellent in film constituents or in the film in-plane uniformity of film thickness than thin films formed by an ion plating method, an electron-beam evaporation method, or a vacuum evaporation method. Therefore, the sputtering method has been widely employed to produce metallic thin films which are used as wiring materials of an integrated circuit, to produce thin films of, for example, a dielectric layer for insulation, or to produce thin films for optical reflection/absorption in optical recording mediums (optical disks) or LED elements as mentioned above.
In general, a target which is used in the sputtering method is produced by obtaining a metal material having a desired element composition according to a dissolving/casting process. However, in an Ag-based alloy target which is used to form Ag-based alloy thin films, there is a problem that Ag and alloy elements other than Ag each of which has a higher melting point than Ag are not sufficiently mixed together when these are dissolved. For example, in an Ag—Ta—Cu alloy target, since the melting point (2977° C.) of Ta is much higher than the melting point (961° C.) of Ag and the melting point (1083.4° C.) of Cu, Ta is not easily melted and it is difficult to produce a target having a uniform element composition. Therefore, when an Ag-based alloy target including Ag and elements each of which greatly differs in melting point from Ag is produced, or when an Ag-based alloy target including Ag and elements each of which is separated from Ag in a liquid phase even if these are melted is produced, it is recommended to produce the Ag-based alloy target according to a powder sintering method in which the powder of each element is used as a raw material.
A target obtained by producing a metallic mixture according to the powder sintering method and then machining the mixture into a target shape is used for sputtering. Generally, an operation called a “pre-sputter” is performed in an initial step of a sputtering process. This “pre-sputter” is a sputtering operation performed to, for example, remove dirt from the surface of a target because sputtering is unstable for the dirt adhering to the surface of the target at the beginning of the use of the target, and the properties of thin films formed as a result are unstable.
However, in an Ag-based alloy target produced by the powder sintering method, in case where the target which is subjected to no treatment after machining is used for sputtering, the number of pre-sputter operations is greatly increased, and the problem of inhibiting the improvement of productivity occurs.
Presumably, this problem is caused by the following two facts (a) and (b). (a) In a target produced by the powder sintering method, there is a greater difference between the distribution state of alloy elements of a machined/altered layer formed on a treated surface of the target by machining and the distribution state of alloy elements inside the target than in a target manufactured by melting/casting method. (b) The sputtering rate of alloy elements in the target is smaller than that of Ag.
For example, the following techniques have so far been proposed as a method of reducing the number of pre-sputter operations.
JP-T-2002-511115 discloses that the surface of a target is pre-processed, and the removal processing time to remove contaminant and/or impurities from the surface of the target when the target is used is shortened. Although chemical polishing or the like is mentioned as a specific method for target surface treatment (see claim 6 in “claims”), a specific means therefor is not described.
Japanese Patent No. 2818206 discloses a target which contains metal having a high melting point and has a surface layer which is subjected to electrolytic polishing in a to-be-sputtered surface of the target. More specifically, “Means and operation for solving the problem” of this patent document discloses that in a target containing metal having such a high melting point, a processed defective layer is formed on the processed surface when the target is machined and ground into a target shape; that if sputtering is performed by using a target having such a processed defective layer on entire surface thereof, ions collide with each other in plasma, and, as a result, fine particles are separated from parts of the processed defective layer which functions as a starting point, of the surface of the target, whereby such particles disadvantageously mixed into a thin film in the form of particles; and that the surface layer of a surface to be sputtered is subjected to electrolytic polishing as a method of swiftly and effectively removing the processed defective layer.
JP-A-1-215974 discloses that the surface of a target is etched by dry etching (see claims). JP-A-2000-169960 discloses that wet grinding or chemical polishing is preferably performed to remove a layer (treated layer or machined/altered layer) still having treatment deformation or oxidation of a target surface caused during finish machining or treatment (see claim 4 in “claims”). However, these patent documents do not disclose a detailed description of such grinding or polishing.
JP-A-1-215973 discloses that the surface of a target is etched by wet etching (see claims). The embodiment therein shows that the surface of a Tb—Fe—Co target is soaked in a nitric acid solution for one minute and that the surface of a Gd—Tb—Fe—Co target is soaked in a buffer-fluorinated-acid solution in which fluorinated acid and ammonium fluoride are mixed together for three minutes.
Any of the techniques disclosed in JP-T-2002-511115, Japanese Patent No. 2818206, JP-A-1-215974, JP-A-2000-169960, and JP-A-1-215973 aims to reduce the number of pre-sputter operations. However, these patent documents do not resolve the problem caused when elements each of which greatly differs in sputtering rate from Ag are added as alloy elements. Therefore, even if the techniques of these patent literatures are applied to the target of the present invention, it is considered to be difficult to reduce the number of pre-sputter operations and shorten the pre-sputter time.